A Demon Found — Holley Trent

A Sons of Gulielmus story set between A DEMON IN LOVE and A DEMON BEWITCHED. Meet Jason -- the newest unwilling recruit in the demon Gulielmus' troop of defiant incubi.
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Half-demon psychic Jason Deiulio has desired grocery store cashier Cam Evans for two years. Normally, he's got swagger in spades, but Cam makes him clam up. No way would a good girl like her crawl into bed with an abomination like him.

Studious Cam is chock-full of surprises, though. She sees supernatural beings few other humans know exist and pegged Jason as a demon from the moment she first saw him. She wants him anyway, and decides it's up to her to make the first move. She does, and not a moment too soon.

Jason's long-absent incubus father pops in to make Jason an offer he can't refuse, and the duo will have to test the mettle of their budding relationship. Cam has no reservations about being the girlfriend of a cambion, but she didn't expect that taking the gig would put her on the supernatural hit list!
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Also available in the Sons of Gulielmus series:
1) A Demon in Waiting
1.5) A Demoness Matched (in the Melt My Heart anthology)
2) A Demon in Love
3) A Demon Bewitched
3.5) An Angel Fallen